FIG. 1 illustrates a configuration of an antenna for a conventional cellular phone. An unbalanced feeding antenna in FIG. 1 is provided with a circuit substrate 11 and an unbalanced feeding antenna element 12.
The unbalanced feeding antenna element 12 operates as an exciter to excite the circuit substrate 11 rather than an antenna. Therefore, an antenna current flows into the circuit substrate 11, which makes the circuit substrate 11 dominant as the antenna. FIG. 2 shows a radiation characteristic using this unbalanced feeding antenna.
FIG. 2 shows a radiation characteristic of a conventional antenna for a cellular phone. Suppose the size of the circuit substrate 11 is 146×45 mm, the length of the unbalanced feeding antenna element 12 is 32 mm and its frequency is 2 GHz. In this case, radiation characteristics of Eφ and Eθ on a free space horizontal plane (x-y plane: see the coordinate axis in FIG. 1) are as shown in the figure and Eθ shows almost no directivity because the circuit substrate 11 is operating dominantly as the antenna.
However, the problem of the cellular phone using the above-described conventional antenna for a cellular phone is that it is easily influenced by the user, resulting in a reduction in the gain. That is, assuming that the user 21 uses a cellular phone 22 as shown in FIG. 3, the circuit substrate 11 is operating dominantly as an antenna, but it is greatly influenced by the user's hand or body, etc., and the radiation characteristic when the user is operating the cellular phone is as shown in FIG. 4. In FIG. 4, the user exists in the direction indicating 270° from the origin and it is observable that the gain has decreased drastically compared to the radiation characteristic in FIG. 2.
Furthermore, FIG. 5 shows a diversity antenna used for a cellular phone and is provided with a circuit substrate 11, an unbalanced feeding antenna element 12 and a dipole antenna 31. The diversity antenna is constructed of the unbalanced feeding antenna element 12 and dipole antenna 31 and has the same problem as that described above when the unbalanced feeding antenna element 12 is operating.